The present invention relates to an electric power steering systems for a motor vehicles, and particularly to an electric power steering system having a system for automatic return of the steering wheel to the straight ahead position and other control functions.
There have been proposed and developed various kind of hydraulic power steering. A typical hydraulic power steering system comprises an engine driven pump, a flow control valve for controlling the flow rate of the hydraulic fluid delivered under pressure from the pump, a steering control valve, and an actuator having a hydraulic cylinder for steering the wheels of the vehicle by pressurized fluid through the steering control valve. Japanese Utility Model Publication No. 53-44181 discloses an example of such hydraulic power steering system.
Difficulties have been encountered in applying some additional steering control functions into the hydraulic power steering system of the above fundamental configuration. Such additional functions include the automatic return of the steering wheel to the straight ahead position when no turning effort is exerted thereon, and the control of the turning effort to be applied to the steering wheel in accordance with the traveling speed of the vehicle or to lateral acceleration. Conventionally, such additional change requires major modifications of the hydraulic piping and the addition of actuators and valves into the systems and results in large increase of the cost. The resulting steering systems have become much more complex and expensive than when no such additional means are included.